This study was carried out in a field solar drier at the North of Portugal, Mirandela (Direcç~ao Regional de Agricultura de Tr_as-os-Montes; Fig. 1). According to the classification of Fuller (1993) , this is a mixed mode or hybrid cabinet dryer. The solar dryer had a collector for pre-heating the air, a drying chamber and a solar chimney. It was made of wood with a polyethylene film cover (Araújo et al., 1994); it was 8.10 m long, 7.50 m wide and 2.00e2.60 m high. The dryer collector was faced south for maximising solar radiation and formed an angle of 38_, which was similar to the local latitude (f) corresponding to 41.28 _N. It had a 30 cm opening throughout all its length, for air entrance. In this area the air was pre-dried before moving to the dehydration chamber. The drying chamber comprised 18 (6 _ 3) sets of 5 trays each (90 trays total). Two exhaust air fans were placed on the back wall. A simplified scheme of the solar dryer is included in Fig. 2.